The Molly Marie Prospect is a group of 20 contiguous mining claims located in Maricopa County, Arizona.  This website describes the exceptional geologic features of the Prospect that includes a very large IOCG (Iron Oxide Copper Gold) ore deposit located in a previously unrecognized submarine caldera over one mile in diameter.

Below is a photo showing the extents of the massive iron-oxide altered breccias of the deposit before they dive under basalt to their east.  The abundant hematite breccias on the hill or bulge in the basalt labeled “Cerro Negra” are not indicated, but are described within.  Please note the highway located approximately 600 yards to the west of the caldera.  The igneous rock exposed at the neck is  an oxidized magnetic, brick-red, Rhyolite porphyry; abundant iron-rich rock is a prerequisite for an IOCG.
The Molly Marie Collapse Caldera

Accompanying this deposit, located near the foot of the Superstition Mountains, is a subject that must be breached:

A “missing” mining district, The Peralta Mines.

The location of these legendary mines has commonly been thought to be deep inside the Superstition Wilderness area, but it’s just outside its boundaries.  All of the stories told about lost mines in the Superstitions during the past 125 years have one thing in common; they do not have any geology to back them up, on  a district scale.  The geology of the District shown within proves the existence of the IOCG deposit, but proof that the District was mined by the Spanish and Mexicans (also shown within),  demonstrates that the IOCG deposit is the most sought-after kind………. an exceptionally  gold-rich one.

The root-cause of the richness of the deposit is likely the rock exposed at the volcanic neck, and what should comprise the magma chamber below: a  pre-enriched porphyry.  The below photo shows what a portion of the volcanic neck looks like.  The color of the porphyry betrays its high iron content.

Rhyolite Porphyry of the Molly Marie caldera

Proof that the Spanish and Mexicans mined the District also helps explain why a deposit so immense could go undiscovered; all of the most obvious outcrops indicating ore were simply removed and the pits (and shafts) were filled back in;  intense alteration zones still betray their locations.

The nearby Goldfield mining District, 2-1/2 miles away, has geologic features similar to that  of the Molly Marie Prospect, including a caldera with breccias on its margin.  Significant gold ore was mined there, in breccias, that assayed hundreds of ounces of gold to the ton;  the volume of breccias at the Molly Marie caldera is multitudes greater than those at Goldfield.

The rarity of this type of IOCG deposit, the magmatic hydrothermal type (MH-IOCG), almost completely intact and exposed at the surface as shown below, cannot be overstated.

Below is a hand sample of rock from the Molly Marie that is exposed in a wide zone approximately 400 yards long in the massive breccias that is aligned with a major fault. The quartz there is very flaky and has the remains of pyrite and chalcopyrite.
Foliated Quartz
Foliated, Flaky Quartz and Jarosite & Hematite

Below is a larger sample, with more manganese mineralization and even heavier remnants of pyrite and chalcopyrite.  Depending on your browser, click and zoom-in to see the mineralization.

Flaky Quartz, Hematite, and Manganese

Below is an outcrop of the massive zone of foliated, flaky quartz, looking very benign, but one hammer blow reveals its secrets.

Flaky Quartz outcrop, peeking out from cover

The foliated quartz was core drilled with a backback drill 50′ deep.  The core recovery was  terrible in the flaky material, but some solid core was recovered.  Unfortunately, the supergene enrichment zone was not reached.  Below is a piece of core from 20 feet deep.

Core from 20′ deep

From 35′ deep in the hole, the acid bleaching of quartz can be seen in this piece of core shown below.
At 35′ deep, acid bleached quartz

Other quartz outcrops in the breccias on the prospect are spotted with blebs of manganese, another testament that brine was instrumental in forming the deposit.

Manganese-spotted quartz

Abundant dimpled float as shown below is more IOCG material. One crack of the hammer reveals the spheres of hematite inside.

Dimpled Float

Similar to vents and mounds normally associated with VMS deposits, below is a nearly fully intact hydrothermal mound of silica heavily inundated with Jarosite on the northern end of the Prospect.  Most of this mound is highly magnetic (attracts a magnet). There is another mound of this material that can be seen to the left in the photo (to the south).  There is recent evidence that there can be as much as 5 times the volume of the mound below the sea floor for similar VMS mounds. Please note that there are two large mounds here, but as shown in the conclusion, there is an entire FIELD of hydrothermal silica mounds exposed on the surface on the Molly Marie Prospect.
Silica/Jarosite mound

Below is an exposure of silica beds on the south side of the north mound.  All of this material is very magnetic. The south mound has similar beds “draped” around it.
Silica beds

Below is a photo of sawn silica that comprises the two mounds; they are nearly totally composed of this silica and Jarosite.   With a 20x loupe, in the sunlight, the true magnificence of the iridescent Jarosite (with many cubes) can be seen.
Sawn Silica with Jarosite (lead is .5mm)

The sawn hydrothermal silica below is from one of the smaller mounds that lie on top of the basalt.  The hematite and jarosite clasts are much larger than the silica found from the large mounds.  The cloudy, delicate deposition bands can be seen.  The silica has a green tinge and in the vugs, chlorite can be seen.  With a loupe, the multitude of grains of magnetite are visible.
Hydrothermal Silica from a smaller mound

If the reader has any doubt that this is hydrothermal silica, a simple test removes all doubt:  When this material is crushed and panned and the powdered hematite is washed out, the pan looks like it is full of only black sand.  But, when the grains are viewed with a loupe, they are grains of clear silica coated with hematite and manganese. Below is a photo of the rock after it was crushed, after it was washed, and a close-up of the nearly 100% quartz left behind.
The crushed hydrothermal silica after being crushed, washed and magnified.

Below is what one of the smaller mounds looks like that is partially eroded.  The material in the smaller mounds is typically much softer than that found on the larger, probably due to the larger fragments.   That’s Superstition Mountain in the background.
Eroded Small Mound

At the top of the large north mound several small dikes of Basanite can be seen.  This attests to the hot spot induced system and its great depth.  Note: this is toughest rock type that the author has ever broken.

Exposed in a wash on the southeastern side of the south hydrothermal mound is a large zone of pyrite altered dacite. This is likely metasomatic alteration from the large hydrothermal system.
Pyrite zone

On the east side of the northern mound, there is gossan found from a sulfide vein.  This vein has a strike length of about 50 feet.  Note that the full width would double if the surface material was removed.  This is evidence that the silica shown above is just a classic cap on the hydrothermal mound and below it are great amounts oxidized ore.
Gossan from a Sulfide Vein

Spectacular bleaching patterns can be seen in the silica as shown in the photo below. The magnet on a backward slope is a testament to the magnetism.

Shown below, on the south side of the southern Silica/Jarosite mound, there is a large area that is covered by friable exhalitive “dirty” silica beds that must have covered the entire area at one time.  This material contains much magnetite and has significant carbonates as well.
Exhalative Silica beds

A very large outcrop has been found that is believed to be the core of a large mound or the mineralization below it.  A photo of a piece of it is below.  It is not very exciting to look at and that must be the reason there are not too many photos of IOCG ore on the internet.  There is abundant waxy yellow apatite and tiny chalcopyrite crystals that can be seen on it with a loupe.  It is very dense and magnetic.   The outcrop is hundreds of feet long and guessing about 200 feet wide.  A couple of large chrysocolla-saturated rocks were found in this area over the past few years, and this must be the origin of the material.  The outcrop is near the a large mound shown above.

IOCG “ore”

More geology, much history, the infrastructure of  the historic District, and the potential for great wealth left to be unearthed is shown within.

After nearly 20 years of study and fieldwork, there is now enough evidence to present the mines that used to be, and where new mines could be.

Select any of the pages in the menu to begin……..


(cover photo is a view of the Superstition Mountains from the southern end of the Molly Marie Prospect)